It can take a long time for the effects of a DVT to wear off and so you'll likely be a on a "blood thinner" for months. Often it's Eliquis and its VERY effective at clot prevention which means your are prone to bleeding. Hitting your head against the board could easily cause a brain bleed. I'd wait till the ultrasound is totally clear and you are off the meds prior to a surf trip. Meanwhile pick up some WoundSeal off Amazon. Great, hospital grade hemostatic product that works on anti-coagulated blood. Very effective for cuts and scrapes.